Controlling lateral two-dimensional electron hole gas hemt in type iii nitride devices using ion implantation through gray scale mask

ABSTRACT

A high electron mobility field effect transistor (HEMT) includes a two dimensional electron gas (2DEG) in the drift region between the gate and the drain that has a non-uniform lateral 2DEG distribution that increases in a direction in the drift region from the gate to the drain.

CROSS REFERENCE TO RELATED APPLICATIONS

This application is related to U.S. patent application Ser. No. ______filed on the same date as this application and entitled “HEMT GaN Devicewith a Non-Uniform Lateral Two-Dimensional Electron Gas Profile andProcess for Manufacturing the Same” (attorney docket no. 626622-2) andto U.S. patent application Ser. No. ______ filed on the same date asthis application and entitled “Non-Uniform Two-Dimensional Electron GasProfile in III-Nitride HEMT Devices” (attorney docket no. 626244-8).

TECHNICAL FIELD

This disclosure relates to type III—nitride HEMT devices and inparticular to two dimensional electron gas (2DEG) in the drift region.

BACKGROUND

A high electron mobility transistor (HEMT) is a field effect transistorincorporating a junction between two materials with different band gaps(i.e., a heterojunction). Gallium nitride (GaN) HEMTs have attractedattention due to their high-power performance. In type III-nitride HEMTdevices used in power applications there is a design trade-off betweenthe on-state resistance and breakdown voltage (BV). Since the relationbetween the BV and on resistance is at least quadratic, improvement inthe BV for a given drift region length results in a significantimprovement in the FOM of the device, defined as BV²/Ron.

In the prior art type III-nitride HEMT devices have a uniform 2DEGdensity which results in a peak electric field under or near the gateregion. The electric field distribution tends to be closer to atriangular shape than to the desired trapezoidal shape which reduces thebreakdown voltage per unit drift region length of the device. The use offield plate and multistep field plates are some of the techniques thatare used to improve the electric field distribution. However, fieldplates typically result in multiple peaks and suffer from less thanideal flat field distribution, and may exhibit a saw tooth profile.Field plates also add to the gate to drain capacitance. In addition,process complexity and cost typically increase with the number of fieldplate steps.

U.S. Pat. No. 7,038,253 to Furukawa describes a GaN based device onsilicon (Si) technology which uses a uniform 2DEG profile in the driftregion. Because of the absence of any field shaping technique in theFurukawa device, the breakdown voltage and dynamic on resistance fromdrain to source is limited by a localized increase in the electric fieldunder the gate region thus requiring over design of the device whichdegrades the figure of merit (FOM) that such a device can achieve.

In “High Breakdown Voltage AlGaN/GaN HEMTs Achieved by Multiple FieldPlates” by H. Xing et. Al, a field shaping technique that uses multiplefield plates is described to improve the electric field distribution.However, multiple field plates do not achieve a uniform electric field,may have a saw tooth type distribution, and introduce gate to draincapacitance. Implementing such a device structure also increases devicecomplexity and cost.

What is needed is a significant improvement in the FOM in type IIInitride HEMT devices, and in particular an improvement in the breakdownvoltage for a given drift region length, so that the FOM of the device,defined as BV²/Ron, improves. The embodiments of the present disclosureanswer these and other needs.

SUMMARY

In a first embodiment disclosed herein, a high electron mobility fieldeffect transistor (HEMT) comprises a two dimensional electron gas (2DEG)in the drift region between the gate and the drain that has anon-uniform lateral 2DEG distribution that increases in a direction inthe drift region from the gate to the drain.

In another embodiment disclosed herein, a high electron mobility fieldeffect transistor (HEMT) comprises lattice damage in a drift region of acarrier supply layer between a gate and a drain, wherein the latticedamage decreases in a direction in the drift region from the gate to thedrain.

In yet another embodiment disclosed herein, a method of fabricating ahigh electron mobility field effect transistor (HEMT), the methodcomprises forming a channel carrier traveling layer on a substrate,forming a carrier supply layer on the channel carrier traveling layer,forming a mask layer on the carrier supply layer, the mask layerconfigured to be aligned with a drift region from a gate to a drain, andconfigured to have a lateral variation in a direction from the gate tothe drain, and implanting ions through the mask layer into the carriersupply layer.

These and other features and advantages will become further apparentfrom the detailed description and accompanying figures that follow. Inthe figures and description, numerals indicate the various features,like numerals referring to like features throughout both the drawingsand the description.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 shows the use of a gray scale mask to control ion implantation tobe tapered in a drift region in accordance with the present disclosure;

FIG. 2 shows a tapered two dimensional electron gas (2DEG) chargedensity in a type III Nitride device in accordance with the presentdisclosure; and

FIGS. 3A-3C are flow diagrams for methods of fabricating a HEMT devicein accordance with the present disclosure.

DETAILED DESCRIPTION

In the following description, numerous specific details are set forth toclearly describe various specific embodiments disclosed herein. Oneskilled in the art, however, will understand that the presently claimedinvention may be practiced without all of the specific details discussedbelow. In other instances, well known features have not been describedso as not to obscure the invention.

Referring now to FIG. 1, a field effect transistor (FET) devicestructure 10 is shown. The FET device structure 10 is composed of astack of III-V layers, such as GaN layer 14 and AlGaN layer 16, grown ona substrate 12 that can be any of the suitable substrates that arecommonly used to grow type III-nitride materials. Suitable substratesinclude but are not limited to silicon (Si), Sapphire, silicon carbide(SiC), and bulk single crystal gallium nitride (GaN).

The stack of III-V layers may include a buffer layer of GaN or aluminumgallium nitride (AlGaN) grown on the substrate 12. Then a channel layeralso known as a channel carrier travelling layer, such as GaN layer 14,is grown on the buffer layer. Then a barrier layer also known as acarrier supplying layer, such as AlGaN layer 16, is grown on top of theGaN layer 14. An AlN spacer layer may be between the GaN layer 14 andthe AlGaN layer 16 to improve device electrical performance.

On top of the AlGaN layer 16 a suitable masking layer 50, which may beSi₃N₄, is grown. The masking layer 50 is used as a masking layer to stopthe majority of the ions implanted via ion implantation 52 from reachingthe AlGaN layer 16. Only a small fraction of the implanted ions, thetail of the Gaussian distribution, are intended to reach the AlGaN layer16 to cause damage to the lattice. The small fraction of ions thatsucceed in reaching the AlGaN layer 16 ideally do not penetrate deepinto the AlGaN layer 16.

Further, the masking layer 50 is configured to vary the density of ionsimplanted along the drift region between a gate and a drain of a fieldeffect transistor (FET). A mask layer 50 may be used, as shown in FIG.1, to form a tapered mask layer 60 in the drift region between points 62and 64. The tapered mask layer 60 has a lateral profile and has a heightthat increases towards the drain. In another embodiment a mask may beused that has with various size openings to vary the density of ionsimplanted along the drift region. Either type of mask modulates the ionimplantation between points 62 and 64, such that lattice damage due toion implantation in the AlGaN layer 16 is greater at the point 62, nearthe edge of gate region 22, as shown in FIG. 2, and less at point 64along the drift region towards the drain 20, as shown in FIG. 2. Themask layer 50 may be configured to provide a lattice damage thatlinearly increases from point 64, along the drift region from near thedrain 20, to point 62 near the gate 22. After ion implantation themasking layer is etched and removed.

The source contact 18 and drain contact 20 shown in FIG. 2 may be formedby metal evaporation or metal sputtering. Then a passivation layer 24may be deposited between the source 18 and the drain 20.

A gate region is then formed by etching through the passivation layer 24in a gate area between the source 18 and drain 20 and into the AlGaNlayer 16. In another embodiment the etch may extend through the AlGaNlayer 16 and partially into the GaN layer 14 to an appropriate depth. Agate dielectric 26 is then deposited over the area between the source 18and gate 22 and the gate 22 and the drain 20, and also deposited to linethe etched trench that extends into the AlGaN layer 16. If the etchedtrench extends into the GaN layer 14, then the gate dielectric 26 alsolines the etched trench that extends into the GaN layer 14.

After deposition of the gate dielectric 26, gate metal 22 is formed byevaporation or sputtering and fills the etched trench.

Various alternating passivation and metallization layers may be formedas a part of back-end processing to improve the parasitic resistance ofthe device and provide connection to device pads and/or a package.

The use of the mask layer 50 with tapered mask layer 60 or in anotherembodiment a mask with various size openings to control ion implantationand thereby the distribution of lattice damage in the drift region ofthe AlGaN layer 14 provides a significant improvement of the figure ofmerit (FOM) in type III Nitride HEMT devices by achieving flat electricfield distribution in the drift region between the gate 22 and the drain20. By controlling the ion implantation and thereby the lattice damagein the drift region from the gate to the drain, the 2DEG 42 is varied inthe drift region to form a non-uniform lateral 2DEG distribution 44. Asshown in the embodiment of FIG. 2, the 2DEG increases in the driftregion in the direction from the gate 22 towards the drain 20. A flatelectric field distribution results from the non-uniform lateral 2DEGdistribution 44 along the drift region which provides the improvement inthe figure of merit (FOM).

Implementing a non-uniform lateral 2DEG profile 44 along the driftregion by causing tapered lattice damage to the carrier supplying layer,such as AlGaN layer 16, controls the level of damage or stress in thatlayer. The lateral control of the damage in the carrier supplying layeris achieved by means of ion implantation of a suitable ion speciethrough a masked layer, such as mask layer 50, that has a taperedprofile, where the vertical height of the masking layer determines thestopping power of the implanted projectiles and hence their projectedrange. The tapered profile of the masking layer may be produced by grayscale photolithography followed by an etch step.

Alternatively the stress in the AlGaN layer 16 may be varied by openingwindows in the photo resist with varying size where the size of theopening is a function of the lateral distance from the gate to thedrain. The size of the openings may be larger or smaller in the driftregion near the gate and decrease or increase, respectively, in thedrift region in the direction of the drain.

Since the density of charge in the 2DEG region is determined locally bythe magnitude of the damage induced by ion implantation, a non-uniform2DEG distribution 44 is achieved by varying the lattice damage laterallyover the drift region. If the lattice damage caused by ion implantationis reduced as a function of a distance from the gate region along thedrift region by increasing the height of the mask layer 50 or byreducing the size of openings in the mask, the 2DEG 44 density increasesas a function of distance from the gate region along the drift region,as shown in FIG. 2.

FIGS. 3A-3C are flow diagrams for methods of fabricating a HEMT a typeIII Nitride device in accordance with the present disclosure.

In step 100 a channel carrier traveling layer 14 is formed on asubstrate 12. Then in step 102 a carrier supply layer 16 is formed onthe channel carrier traveling layer 14. In an embodiment, the layers 14and 16 are formed by an epi manufacturer.

Next in step 104 a mask layer 50 is formed on the carrier supply layer16. The mask layer is configured to be aligned with a drift region froma gate to a drain, and configured to be have a lateral variation in adirection from the gate to the drain. Then ions 52 are implanted throughthe mask layer 50 into the carrier supply layer 16.

In one embodiment the mask layer is formed in step 108 by forming atapered section on the mask layer that has a thickness that increases inthe direction from the gate to the drain by using gray scalephotolithography and then in step 110 etching the mask layer to form thetapered section.

In another embodiment the mask layer is formed by coating the carriersupply layer with photoresist in step 112 and then in step 114 openingwindows in the photoresist of varying size such that the size of theopenings decrease in the direction from the gate to the drain.

Having now described the invention in accordance with the requirementsof the patent statutes, those skilled in this art will understand how tomake changes and modifications to the present invention to meet theirspecific requirements or conditions. Such changes and modifications maybe made without departing from the scope and spirit of the invention asdisclosed herein.

The foregoing Detailed Description of exemplary and preferredembodiments is presented for purposes of illustration and disclosure inaccordance with the requirements of the law. It is not intended to beexhaustive nor to limit the invention to the precise form(s) described,but only to enable others skilled in the art to understand how theinvention may be suited for a particular use or implementation. Thepossibility of modifications and variations will be apparent topractitioners skilled in the art. No limitation is intended by thedescription of exemplary embodiments which may have included tolerances,feature dimensions, specific operating conditions, engineeringspecifications, or the like, and which may vary between implementationsor with changes to the state of the art, and no limitation should beimplied therefrom. Applicant has made this disclosure with respect tothe current state of the art, but also contemplates advancements andthat adaptations in the future may take into consideration of thoseadvancements, namely in accordance with the then current state of theart. It is intended that the scope of the invention be defined by theClaims as written and equivalents as applicable. Reference to a claimelement in the singular is not intended to mean “one and only one”unless explicitly so stated. Moreover, no element, component, nor methodor process step in this disclosure is intended to be dedicated to thepublic regardless of whether the element, component, or step isexplicitly recited in the Claims. No claim element herein is to beconstrued under the provisions of 35 U.S.C. Sec. 112, sixth paragraph,unless the element is expressly recited using the phrase “means for . .. ” and no method or process step herein is to be construed under thoseprovisions unless the step, or steps, are expressly recited using thephrase “comprising the step(s) of . . . . ”

1-12. (canceled)
 13. A method of fabricating a high electron mobilityfield effect transistor (HEMT), the method comprising: forming a masklayer on a carrier supply layer, the mask layer configured to be alignedwith a drift region from a gate to a drain, and configured to have alateral variation in a direction from the gate to the drain; andimplanting ions through the mask layer into the carrier supply layer.14. The method claim 13 further comprising: forming a channel carriertraveling layer on a substrate; and forming a carrier supply layer onthe channel carrier traveling layer.
 15. The method of claim 13 whereinforming a mask layer on the carrier supply layer comprises: forming atapered section on the mask layer that has a thickness that increases inthe direction from the gate to the drain by using gray scalephotolithography; and etching the mask layer to form the taperedsection.
 16. The method of claim 13 wherein forming a mask layer on thecarrier supply layer comprises: coating the carrier supply layer withphotoresist; and opening windows in the photoresist of varying sizewherein the size of the openings decrease in the direction from the gateto the drain.
 17. The method of claim 13 wherein implanting ionscomprises: damaging a lattice of the carrier supply layer; wherein thelattice damage decreases in a direction in the drift region from thegate to the drain.
 18. The method of claim 17 wherein: the substratecomprises silicon (Si), sapphire, silicon carbide (SiC), or bulk singlecrystal gallium nitride (GaN); the channel carrier traveling layercomprises a GaN layer; and the carrier supply layer comprises a AlGaNlayer.
 19. The method of claim 18 further comprising: forming apassivation layer over the AlGaN layer; and forming a gate comprisingthe steps of: forming a trench extending through the passivation layerand into the AlGaN layer; and depositing a gate dielectric layer in thetrench; and forming gate metal on the gate dielectric.
 20. The method ofclaim 19 wherein forming a trench extending through the passivationlayer and into the AlGaN layer further comprises: forming the trench toextend into the GaN layer.
 21. The method of claim 19 wherein thelattice damage decreases in a direction in the drift region from thegate dielectric layer to the drain.
 22. The method of claim 17 whereindamaging a lattice of the carrier supply layer further comprises:forming a two dimensional electron gas (2DEG) in the drift regionbetween the gate and the drain that has a non-uniform lateral 2DEGdistribution that increases in a direction in the drift region from thegate to the drain.